Making better liming decisions

Take home messages

• Sampling in 10 cm increments to 30 cm gives a good guide to acidity within the soil profile and will pick up any formation of acidic layers.
• Lime to maintain a good soil pH profile, pHCa above 5.0 in the topsoil or 5.5 when there are subsurface acidity issues and pHCa greater than 4.8 at depths below 10 cm.
• Maintenance liming is needed rather than waiting for soil pH to fall to levels where you get a response because lime takes time to neutralise soil acidity and responses are a reflection of lost production.
• Surface applied lime is not affected by burning stubble but bare paddocks could lead to lime being lost through wind removal.

Background

Soils in most parts of Victoria are generally acidic and are slowly becoming more acidic over time due to weathering and farming practices. Agriculture increases acidification rates and as we continue to increase productivity our soils will start to acidify more quickly. Current rules of thumb of applying one lime tonne/acre every ten years may not be enough to keep pace with our acidification rates. 

SFS are working with producers on three soil acidity related projects that are funded by GRDC and supported by the Corangamite Catchment through funding from the Australian Government. They are:

1. Soil acidity monitoring within the Corangamite Catchment.
2. Trials to measure crop responses to liming in South West Victoria so that simple lime response calculators can be created to help growers make better liming decisions.
3. Trials to measure pasture responses to liming and biological amendments within the Corangamite catchment.

These projects will help determine the amount of lime needed in different soil types to remove production constraints, recover soil pH to desirable levels and answer questions such as how long before I need to reapply lime. They will also provide financial information on the costs and returns of maintaining a good soil pH profile from liming. 

Acidity: the basics

Soil pH is a measure of how acid or how alkaline the soil is. There are two main pH tests using either solutions of calcium chloride or water. In this paper, pH results presented are using calcium chloride as it removes some of the seasonal variation that can be caused by moisture. Generally pH results from the water test are 0.7 units higher than the calcium test. Even small decreases in pH results are important as they result in large increases in acidity because the pH scale is logarithmic. For example a soil with a pH of 4.5 is 10 times more acidic than a soil with a pH of 5.5 and 100 times more acidic than a soil with a pH of 6.5.

Soil acidity can vary throughout the soil profile and in SW Victoria pH commonly increases as you go down in depth. We normally soil test the 0-10 cm fraction of topsoil which generally goes down to 20 to 25 cm before organic matter declines and the subsoil begins. Deeper soil testing has always been recommended when establishing acid sensitive species such as lucerne because rooting depth is critical for its growth and survival over summer. In this project, soils were tested in 0-10, 10-20 and 20-30 cm increments to find out how acid the topsoils are, if acidity layers are forming and if subsoil acidity is present.

As a minimum, pH_Ca should be at least 5.0. However, for soils with subsurface acidity issues, a pH_Ca of at least 5.5 is needed to allow enough lime to treat any acidity occurring below 10 cm. Research has shown that when the topsoil pH is below 5.5 any lime applications or alkalinity deposits from decaying plants is used up neutralising the acidity within the top 10 cm of soil and none is available to move into and treat the subsurface acidity (Gazey et al, 2014). Soil pH for depths below 10 cm should be at least 4.8 to avoid aluminium toxicity issues.

There are many negative effects on plant growth and soil biology, fertility and structure when soils become too acidic. One major effect is when pH falls to 4.8. At this point aluminium starts to become more soluble where it is toxic to plants and restricts their root growth and function. Plant species have different tolerances to soluble aluminium. There are four broad categories of plant tolerance (Highly sensitive, Sensitive, Tolerant and Highly tolerant). Plants highly sensitive to aluminium have their yields effected at low levels of aluminium (approximately Al % CEC > 2%). When pH falls below 4.5, the amount of aluminium increases markedly and even plants tolerant of aluminium will suffer yield reductions or fail to persist.

The aluminium test used in this program reflects the amount of aluminium within the soil solution which roots are exposed to and is expressed as a percentage of cations. When interpreting the results, the electrical conductivity (EC) which is a measure of salts within the soil is needed to accurately predict the effect of aluminium on plant growth.

Methodology

In the soil monitoring project fifteen cores were collected in each paddock across a linear transect of 150 m. Each core was divided into three depths (0-10, 10-20 and 20-30 cm) and bulked together for testing. Tests included basic soil fertility, soil acidity and fractionated carbon but only the 2014 soil acidity results are reported below. Sites included both pasture and cropping, with meat, wool, cropping and dairy enterprises represented.

Eight crop and fourteen pasture lime response trials were established across SW Victoria and in addition SFS took over the long term monitoring of seven lime trials set up by the Woady Yaloak catchment group. Sites were chosen that had very little recent lime history and were likely to be acidic. Trials are a randomised block design with eight to ten treatments replicated four times. Plot sizes were four by 14 m. Six different rates of lime (0, 500, 1250, 1750, 2500, 3750 kg/ha) were applied to develop response curves. A soil fertility treatment was also applied in the crop and pasture sites with and without lime aiming at removing constraints to production.  Up to two biological product treatments were also included at the pasture sites.

A further trial site was established on the Bellarine Peninsula to investigate if burning stubble reduced the effectiveness of recently surface applied lime. A randomised block design was used with four treatments including two lime rates (0, 3000 kg/ha) with and without burning which were replicated four times. Plot sizes were four by 14 m. Four metre square cuts were taken in each replicate to estimate stubble loads. Stubble loads were initially calculated to be 9.4 t/ha following a 6 t/ha wheat crop. Lime was applied in mid April and the stubble burnt in mid May due to fire restrictions. By this time, stubble loads had decreased due to early rain and only a cool burn was achieved. The site was sown on 13/6/2014 to Westminister barley. Plant establishment counts were taken six weeks after sowing.

Lime movement at the trial was also studied with regular soil sampling. Four 10 cm cores were taken from each treatment and divided into 2.5 cm increments. Treatments across the four replicates were bulked together and their pH tested.

Results and discussion

Soil monitoring

The findings from the Corangamite soil monitoring program are relevant to Gippsland soils even though there are some differences in soil types. Corangamite catchment soils have an issue with surface soil acidity (0-10 cm), with 59% of the soil samples below the preferred level of 5.0 (Figure 1). Acidity in the topsoil will have implications not only for plant growth and soil biology within it but it will also contribute to the development of subsurface acidity. Research has shown that when the topsoil pH is below 5.5 any lime applications or alkalinity deposits from decaying plants is used up neutralising the acidity within the top 10 cm of soil and none is available to move into and treat the subsurface acidity. 

Figure 1. Percentage of sites with different pH ranges within the topsoil (0-10cm) depth.

Given that 90% of topsoil sites had soil pH below 5.5 it’s likely they are too acidic to allow any applied lime to work beyond the 10 cm soil depth and this seemed evident from the soil testing results. Thirty three percent of sites had soil pH lower at the 10-20 cm depth than in their topsoil and strongly acidic layers (10-20 cm) existed in 16% of sites in sedimentary, alluvial and granite soils (Table 1). Further analysis is needed to see how many of these paddocks had been limed. In some cases lime may have been applied recently and had not yet moved deeper into the soil profile. In WA it takes surface applied lime 3-4 years to change pH at depth (Gazey et al, 2014).

Table 1. Percentage of sites where soil pH decreased with depth.

When the soil pH at:	Percentage of sites	Percentage of sites that were ALSO less than pH 4.8
10-20 cm was less than topsoil	33%	16%
20-30 cm less than topsoil	20%	10%
20-30 cm less than 10-20 cm	11%	4%

At these sites root growth may be stunted depending on the crop or pasture species tolerance to aluminium and may show up as uneven crop or pasture growth. Plant roots will aim to find less acidic areas to grow through (termite holes or old root channels) to try and reach better soil. If the roots find the same or more hostile acid conditions at the 20-30 cm depth then root growth will be further stunted depriving plants access to nutrients and moisture at depth.

Figure 2. Percentage of sites with different pH ranges within the 10-20 cm depth.

In 17 percent of the paddocks tested soil acidity issues extended into the subsoil (20-30 cm) (Figure 3). Addressing soil acidity at this depth is difficult because surface applied lime takes time to move to this depth and incorporation can cause inadvertent mixing of low quality subsoil with topsoil. The soils found to be acidic at depth were mainly formed on sedimentary, alluvial, granite or marl based soils and located in high rainfall areas (greater than 600 mm). Soils formed on basalt had no issues with acidity at 20-30 cm as they naturally increase in pH with depth due to their high calcium oxide and magnesium oxide content which can neutralise acidity.

Figure 3. Percentage of sites with different pH ranges within the 20-30cm depth.

Further analysis of deeper cores has been undertaken by DEPI and indicates some soil types and areas have acidity issues extending below 30 cm. The soil types found are roughly coinciding with those mapped in the Australian soil order Kurosols which by definition have subsoil (B horizon) with pH_Ca below 5.4. Soil maps like the West Gippsland Kurosol map shown in Figure 4 can be found in the catchment management regions of Victorian Resources Online (VRO).

Figure 4. A broad map of the location of Kurosol soils in West Gippsland from Victorian Resources Online.

Soil acidity issues below 10 cm are not well recognised as limited soil testing occurs below 10 cm. Sometimes deeper soil pH testing of 10-30 cm is advised but this may not pick up the formation of acidity layers as a soil layer with higher pH would raise the overall pH of the sample. On soil types with an absence of subsoil acidity (e.g. basalt plains soils), pH monitoring could occur at the 0-10 cm and 10-20 cm depths. On other soils increment testing down to 30 cm is recommended. If the 20-30 cm layer is found to have a pH less than 4.8 then further testing down the profile is needed.

Table 2. Median pH and aluminium results for different soil parent materials across the Corangamite catchment with the highest and lowest recorded values shown in brackets.

Soil Parent Material	Tests	Soil Depth (cm)
		0-10	10-20	20-30
Basalt, (29 sites)	pH(Ca) Al % CEC	5.0 (4.3 - 6.3) 1% (0.1 - 12%)	5.3 (4.8 – 7.4) 0.2% (0.1 - 8%)	5.8 (4.9 – 7.6) 0.1% (0.1 – 2.4%)
Sedimentary & Alluvial soils (64 sites)	pH(Ca) Al % CEC	4.8 (4.1 – 6.1) 2.5% (0.1 - 21%)	4.8 (4.2 – 6.2) 2% (0.1 - 29%)	5.0 (4.3 - 6.5) 1% (0.1 - 46%)
Marl (5 sites)	pH(Ca) Al % CEC	5.0 (4.7 – 5.2) 0.5% (0.4 - 5%)	4.9 (4.8 – 5.0) 2% (1 - 8)	4.8 (4.6 – 5.3) 3% (1 - 12%)
Granitic soil (1 site)	pH(Ca) Al % CEC	4.7 4%	4.4 19%	4.9 4%
Limestone soil (1 site)	pH(Ca) Al % CEC	7.2 <0.1%	8.4 <0.1%	8.4 <0.1%

The low pH within the topsoil and subsurface soils has caused aluminium to become available and it was estimated that 51% of sites could be potentially affected by aluminium toxicity depending on the plant species grown and their sensitivity to aluminium. It’s also expected that nitrogen fixation of legumes (subclover, faba beans and lucerne) would decrease as rhizobia survival and function is reduced when their soil pH falls below 5.1. Molybdenum becomes less available to legumes at pH levels below 5.3 and so molybdenum deficiency is likely to be occurring in many of the acidic paddocks. Molybdenum is needed especially for nitrogen fixation and synthesis of amino acids.

Crop lime response trials

Lime applications are needed for maintenance of desirable pH levels and to recover soils when pH levels have fallen. Keeping topsoil pH above 5.5 although highly desirable for lime movement may seem economically unfeasible. So what are the responses of different crops to lime application and is yield being forfeited? One of the first steps in answering this question is to look at the lime responses of crops and pastures. In trial work we want to see responses to get a better understanding of the value of liming, but for the grower a lime response it not a measure of success because it means production has been lost to soil acidity. Where there is no response, the lime is not wasted but acting to maintain a good soil pH profile.

Our crop trial results for 2014 showed some lime responses in acid sensitive crops of canola and barley but not wheat and most were not statistically significant (Table 3). Lime was applied in May 2014 and it is not common to see a lime response in the first year of application. Other reasons for not getting a response may be there are other constraints occurring such as sodicity which lime may also address over time.

Table 3. A summary of soil testing and yield results for crop lime response trials for 2014.

Location	Crop 2014	Depth (cm)	Texture	pH (Ca)	Al% of CEC	Lime response
Bellarine	Westminister	0-10 0-20 20-30	Loam Clay Loam Clay	4.2 4.4 4.9	16.6% 10.7% 2.5%	Lime 3 t/ha increased yield 50% Sig. P= 0.0025, CSV 12.87
Gatum	Canola	0-10 0-20 20-30	Clay Loam Light Clay Light Clay	4.4 4.5 5.4	12.8% 5.5% 0.1%	Lime 2.5 t/ha increased yield 21%. NS
Hensley Park	Revenue Wheat	0-10 0-20 20-30	Clay Loam Clay Loam Light Clay	4.4 4.7 5.3	7.7% 1.7% 0.5%	No treatment responses. NS
Mingay	Westminister Barley	0-10 0-20 20-30	Clay Loam Light Clay Heavy Clay	4.8 5.2 5.7	1.8% 1.4% 0.1%	No treatment responses. NS
Modewarre	Revenue Wheat	0-10 0-20 20-30	Clay Loam Clay Loam Clay Loam	4.6 4.6 5.0	1.4% 2.3% 0.7%	No lime responses. NS. Urea 100 kg/ha at GS 31 increased Protein 13% vs Nil. NS
Shelford	Canola	0-10 0-20 20-30	Clay Loam Light Clay Clay	4.9 5.4 6.0	0.4% 0.1% 0.1%	Accidental windrowing of the trial site. No harvest data available.
Warncoorte	Wheat	0-10 0-20 20-30	Clay Loam Light Clay Heavy Clay	4.5 4.6 5.2	2.1% 2.4% 0.9%	Crop cut for hay due to ryegrass. Treatment responses NS.
Westmere	Revenue Wheat	0-10 0-20 20-30	Clay Loam Clay Loam Clay	4.3 4.7 5.5	3.9% 5.9% 0.1%	Lime 1.75 t/ha increased yield 19% vs Nil. NS. Urea 100 kg/ha increased Protein 12% vs Nil, Sig. P=0.014, CSV 4.63 Urea increased Screening % 50% vs Nil, Sig. P= 0.012, CSV 17
Yulecart	Canola	0-10 0-20 20-30	Clay Loam Clay Loam Light Clay	4.5 4.5 5.0	9.2% 9.3% 0.9%	Yield response 12% at lime 2.5t/ha vs Nil, NS

Only the Bellarine peninsula site showed a significant increase in barley yield by 1 t/ha. This site was extremely acid with high aluminium levels and barley is sensitive to aluminium. Lime movement was shown to be very quick, most likely due to the light textured nature of the soil. Lime movement was down approximately 6 cm after 6 months which is very quick but not quick enough to address soil acidity beyond 10 cm by the time of crop harvest. On 20 paddock trials conducted as part of the SGS program by Cam Nicholson, consultant in SW Victoria, soil pH change was detected down to 5 cm after 16 months. The take home message is to get your lime on well before it is needed.

Figure 5. Soil pH change 7 weeks (June) and then four months (August) after lime application in April 2014.

Figure 6. Soil pH change 6 months (October) after lime application in April 2014.

The Woady Yaloak Catchment group have been conducting lime trials near Ballarat where lime was applied in 2012 and so has had longer to work. They have had responses in canola but not in faba beans, which was expected as it is acid sensitive. Soil nitrogen tests will be undertaken to see if there is more nitrogen available from Rhizobia nitrogen fixation under the lime applications. There is currently no soil test data below 10 cm, and therefore, no explanation as to what is happening at depth. It’s surprising that 1.25 t/ha of lime has been getting yield responses in clay loam soils but not at 2.5 t/ha. It’s questioned whether the higher rate of lime could be inducing a trace element deficiency (copper or zinc) which maybe constraining production. This theory will be further investigated.

Table 4. A summary of soil testing and yield results for Woady Yaloak catchment group lime crop response trials, where lime was applied in 2012.

Location	Crop	Texture 0-10 cm	pH (Ca) 0-10cm	Al% of CEC 0-10cm	Lime response comments
Rokewood	Canola 2012 W. Wheat 2013 Red wheat 2014	Loam	4.8	3%	Canola response lime 1.25 t/ha & 2.5t/ha with 19% & 31% increases, NS. (only just),  P=0.06, CSV 10.2. No response to either wheats
Rokewood West	Wheat 2012 Faba beans 2013 Canola 2014	Clay Loam	4.8	2.4%	No data collected in 2012. Faba beans were cut for hay due to ryegrass. No DM yield response, NS. Canola response lime 1.25t/ha with 16% increase vs Nil but not at 2.5 t/ha. NS
Werneth	Canola 2012 Wheat 2013 Faba Beans 2014	Clay Loam	4.7	2.3%	Canola response lime 1.25t/ha with 8% increase vs Nil but not at 2.5 t/ha. NS. No response in wheat or Faba beans

Burning and liming

Our Bellarine trial also investigated whether stubble burning reduces the effectiveness of recently applied lime. Growers were questioning if they could put out lime prior to burning stubble to try to save time in preparation of crop sowing. Although a significant response was found to liming, there was no significant effect of burning on the crop yield. This could have been due to the one month delay between burning and liming or the occurrence of a cool burn. However, the fact is that lime is not affected by burning. At 900^oC agricultural lime (CaCO₃) is burnt in kilns to create burnt lime (CaO) which makes it faster acting for use in the horticultural industry. Stubble burns might reach 500^oC at most. The only danger with burning stubble is leaving paddocks bare where lime might be blown away. Ideally lime would be spread into standing stubble which will help protect it and dews and rainfall will partially dissolve it and move into the soil before burning occurs.

Table 5. Grain yield and grain testing results of Barley at the Bellarine site with different liming and burning treatments.